The present state-of-the-art for Very Large Scale Integration (“VLSI”) involves chips with circuitry built to design rules of 0.25 μm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (“UV”) delineating radiation. “Deep UV” (wavelength range of λ=0.3 μm to 0.1 μm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 μm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require extreme ultraviolet (EUV) and x-ray wavelengths. Various EUV and x-ray radiation sources are under consideration. There include, for example, (1) the electron ring synchrotron, (2) laser plasma source, (3) discharge plasma source, and (4) pulsed capillary discharge source. Some of the current sources of EUV eject debris that tend to coat optics used in photolithography which ultimately reduces efficiency.
In the next-generation of Extreme Ultraviolet Lithography (EUVL), multilayer based optics and masks will also be subject to carbon contamination. The carbon contamination arises from EUV or plasma-induced dissociation of hydrocarbons absorbed onto optical surfaces from the residual background environment. Although contamination may be minimized by cleaning the vacuum environment, the carbon cannot be entirely removed. Current methods of removing carbon from surfaces are mostly oxidative, in that reactive oxygen species are generated to gasify the carbon into volatile CO and CO2 which can be pumped away.
One challenge in EUVL is that the optics will be buried under layers of surrounding hardware, such as mechanical frames and cabling, as well as mechanical devices used to perform and monitor the lithographic process. A state-of-the-art EUVL machine is described in Tichenor et al., U.S. Pat. No. 6,031,598. The obscuring structures in the machine make it very difficult to direct reactive species generated from the exterior at the tool periphery to the optics located in the interior of the machine. Reactive gas phase species that encounter solid objects can be quenched prior to reaching the optics needing cleaning. Therefore, the art is in need of techniques to generate reactive species inside the optic mounting assembly in a manner that limits adverse effects on the optic.